In the manufacture of semiconductor devices, the formation of conductive lines, for example, digit lines (bit lines) and word lines, is critical to produce a reliably functioning device. The structures, usually manufactured from a layer of polycrystalline silicon (poly), form an X-Y coordinate to select a single storage capacitor from an array.
As devices continue to increase in density, the features which form the various structures on the surface of the die decrease in size. Polycrystalline silicon is a material which is widely used to form the digit lines due to its conductive properties. As the devices increase in density, however, the decreasing width of the poly lines leaves a smaller volume of material to conduct a current, thereby making a read or a write to the capacitor which forms the storage node less reliable. As a result, data errors can occur.
One method which has been used to increase the conductive properties of the digit lines, transistor gates, and active areas on a random access memory, for example, is to use a material more conductive than the poly, such as titanium silicide, to increase the overall conductivity of the line. A typical structure comprising titanium silicide to increase conductance is shown in FIG. 1. To form a typical structure, the source 10 and drain 12 regions are formed in a substrate 14 of single crystal silicon, and the transistor gate 16 is formed over a layer of gate oxide 18. Oxide spacers 20 are then formed. Sputtered titanium reacts with the silicon, both polycrystalline 16 and doped monocrystalline 10, 12, to form a layer of titanium silicide 22A, 22B on the silicon gate 16 and active areas 10, 12. The oxide spacers 20 prevent titanium silicide 22B from forming on the poly gate sidewalls and shorting the poly gate 16 with the active areas 10, 12. Methods of semiconductor manufacture are described in "SILICON PROCESSING FOR THE VLSI ERA", by S. Wolf and R. N. Tauber, 1986, which is incorporated herein by reference.
The titanium silicide greatly increases the conductance of the active areas and the transistor gate. Titanium silicide is known to conduct electricity up to 50 times better than an equal amount of doped polycrystalline silicon.
A conductive line which is even more conductive than the titanium capped line described above would be desirable. This is especially true as devices continue to decrease in size and the conductivity of a polycrystalline silicon line becomes more difficult to maintain past a critical level because of a decreasing volume of material available through which to conduct a charge.